Growth increments and biomineralization process in cephalopod statoliths

A study on morphological, structural and biochemical composition of Sepia officinalis and Loligo vulgaris statoliths and statocyst endolymph was undertaken with the aim of determining the major factors affecting the deposition process of statolith formation and to clarify the cause for the poor definition of the growth increments in S. officinalis statoliths. It is suggested that the different biochemical composition of the statocyst endolymph found in the two species accounts for distinct statolith crystallisation processes, which results in a different microstructure. This explains the better definition of growth increments in L. vulgaris statoliths comparing with those of S. officinalis. The protein content as well as Ca(2+) and Mg(2+) concentrations in the endolymph are more implicated in growth increments formation than Sr(2+) ion concentration. Moreover, the daily variations of the three factors mentioned, allowed us to formulate a working hypothesis to explain the daily deposition of growth increments: a dark ring (rich in organic matter) is deposited during daylight whereas a light ring (rich in CaCO(3)) during darkness. These results are discussed in the light of alternative hypotheses explaining the deposition mechanisms in statoliths.